If it isd a true mystery snail it won't eat your plants. Also, assuming that is wasn't pregnant before you got it then it won't lay eggs either as they need a male and female to reproduce. Very nice snails because of this. If it does lay eggs, they will be above the waterline and you can easily remove them.
If you have an apple snail you will know it. It will deliberately eat you plants and you will see them dissapear in front of your eyes. If you haven't had any noticeable plant munching it is likely you don't have an apple snail and are safe.